This invention relates to processes for the preparation of (substituted-thio)alkyl isothiocyanates.
Isothiocyanates are known to have various utilities depending on what substituents are present. Allyl isothiocyanates are taught to be useful as intermediates in the preparation of thionocarbamates useful in sulfide ore flotation in U.S. Pat. No. 4,479,903 to Dauplaise. Alkyl-thioalkyl isothiocyanates are taught to be useful as spices in U.S. Pat. No. 3,160,649 to Hasselstrom et al.
One of the problems associated with the preparation of these compounds is the difficulty of obtaining exclusively the isothiocyanate isomer rather than the thiocyanate isomer or a mixture of both isomers. Some R-SCN compounds are readily isomerized to R-NCS by the simple addition of heat. This is particularly true when R represents --CH.sub.3 or --CH.sub.2 CH.dbd.CH.sub.2. However, it has been recognized that, when R represents a saturated alkyl radical other than the methyl radical, isomerization does not occur in a practical way merely with the addition of heat. See, e.g., Organic Chemistry of Bivalent Sulfur, Vol. VI, Ch. 1, p. 64, ed. E. E. Reid, Chemical Publishing Co., Inc. (1965).
A study of the synthesis of alkyl-2-thiocyanatoethylsulfides in Uch. Zap. Azerb. Gos. Univ., Ser. Khim. Nauk, No. 1, 50-51 (1972) determined that these thiocyanates are very stable and do not isomerize to the isothiocyanates after heating up to 200.degree. C. for five hours.
It is generally recognized that isomerization to the isocyanate isomer may be obtained in some cases by the use of a catalyst. In U.S. Pat. No. 3,584,028, a method of preparation of organic isocyanates requiring the use of certain halide catalysts, such as alkali metal bromides, alkali metal iodides, and alkaline earth metal bromides, is disclosed. In Organic Chemistry of Bivalent Sulruf, supra, it is taught that certain salts, for example cadmium iodide, will facilitate isomerization of thiocyanates.
Due to the difficulty of isomerizing thiocyanates to isothiocyanates, typical methods of preparing isothiocyanates involve preparing the isothiocyanate directly.
Kjaer and Christensen in Acta. Chem. Scand., 11, 1298-1307 (1957) teach that isothiocyanates corresponding to the formula CH.sub.3 S(CH.sub.2).sub.n NCS (n=2-9) can be prepared from the corresponding chloride in a three-step procedure resulting in a yield of less than 50 percent. Their process involves reaction of the chloride with potassium phthalimide: reduction of this product to the corresponding amine via hydrazine hydrate: and, finally, reaction of the amine with thiocarbonyl chloride to obtain the isothiocyanate.
It is also known that isothiocyanates may be produced from the reaction of acid chlorides and potassium or ammonium thiocyanates.
What is needed is a process for synthesizing isothiocyanates that is simple, uses readily available reactants and gives high yields.